Intervertebral prosthesis especially for a neck vertebral support

ABSTRACT

Intervertebral prosthesis, in particular for the cervical spine, consisting basically of a first cover plate ( 1 ) to be connected to a first vertebral body, a second cover plate ( 2 ) to be connected to the second vertebral body, and a prosthesis core ( 10 ) which is held by a seat ( 5, 6, 7 ) of the first cover plate ( 1 ) and forms an articulation ( 20, 21 ) with the second cover plate ( 2 ). The core ( 10 ) is movable in the AP direction relative to the first cover plate ( 1 ). Mobility can also be provided in the lateral direction and rotational direction.

[0001] Intervertebral prostheses are used for replacing theintervertebral disk. They comprise two cover plates, whose outersurfaces are designed for connection to adjacent vertebral bodies, andan articulation device enclosed by the cover plates. In a knownprosthesis (EP-B 471 821), the upper cover plate forms a concavelyspherical articulation surface on its inner side, which cooperates withthe convexly spherical top surface of a prosthesis core of polyethylenein order to form an articulation. The core has a flat underside and acylindrical edge which are received with matching fit in a seat which isformed on the inside by the lower cover plate.

[0002] In order to permit easy flexion movement, it would be useful tochoose a small radius of curvature of the articulation surfaces.However, this would lead to a small corresponding surface area of thearticulation surfaces and would lessen the capacity for loadtransmission. In said known prosthesis, the radius of curvature of thearticulation surface is chosen approximately equal to half theprosthesis diameter. This has the disadvantage that the prosthesis has aconsiderable structural height and in many cases cannot be accommodatedin the restricted intervertebral space. If, in said type of prosthesis,one were to choose a still greater articulation radius, so that the coreassumed the shape of a flat sphere section, it would be found that thearticulation properties of the prosthesis leave something to be desired.

[0003] The object of the invention is to make available a prosthesiswhich has a small structural height and yet has good articulationproperties, in particular for use in the cervical spine.

[0004] The solution according to the invention lies in the features ofclaim 1 and preferably those of the dependent claims. Accordingly, anintervertebral prosthesis of the type mentioned in the introduction ischaracterized in that the core is movable at least in the AP directionrelative to the cover plate forming the seat. Mobility can also beprovided in the lateral direction and in rotation. The invention isbased on the knowledge that, with a comparatively large radius of thearticulation surfaces, the articulation movement is associated with atranslation movement of the cover plates in relation to one another, theextent of this movement increasing the greater the articulation radius,and that this translation movement is countered by the resistance of theligament apparatus and the facet articulations acting between thevertebrae.

[0005] The invention has recognized that this problem can be overcome byallowing the core to execute a displacement, which compensates for thetranslation offset of the cover plates relative to one another, inrelation to the cover plate holding it. For example, upon a flexionmovement relative to the core, the upper cover plate not only pivotsabout a transverse axis, but also executes a displacement in the ventraldirection. This displacement can be compensated by a correspondingdorsal displacement of the core (together with the upper cover plate)relative to the lower cover plate.

[0006] The invention also has the advantage that the particular relativeposition of the upper and lower cover plates can be adapted to theparticular anatomical conditions. This applies in particular to thosecases where the ventral end faces of the adjacent vertebral bodiesdetermine or influence the position of the cover plates assigned to themand protrude to different extents in the ventral direction.

[0007] The core must be held securely in the prosthesis, so that itcannot protrude into the spinal canal for example. For this purpose,movement-limiting devices, which restrict the extent of the movementwhich the core is allowed, can be provided on one or both cover plates.These movement-limiting devices can interact for example with the outeredge of the core. For example, the lower cover plate can have a raisedcollar which extends completely or partially around it and whichinteracts with the outer edge of the core and is so high that the core,even upon a certain expansion of the intervertebral space, cannot slideover it. As is known per se (DE-C 30 23 353), this edge can also be sohigh in places that it forms projections which engage in correspondingrecesses of the opposite cover plate in order to form a cage forretention of the core.

[0008] The mobility of the core relative to the cover plate holding itis particularly important in the AP direction because the greatestrelative movements (flexion and extension) take place in the sagittalplane, whereas the lateral bending movements are comparatively slight.In an advantageous embodiment of the invention, provision is thereforemade for the movement-limiting device to be designed as a guide devicein the AP direction. In particular, it can be formed by opposite,parallel lateral guide rails between which the core is held in such away that it can move only in the AP direction. The guide rails are inthis case expediently undercut in order to interact with a ridge of thecore which engages in the undercut. In this way, it is ensured that thecore does not lift from the cover plate holding it. This has theadvantage that the devices provided for restricting the movement of thecore do not have to be very high and, for this reason, there is also norisk that they could impede the relative movement of the cover plateswith respect to one another. To ensure that the core does not slide outfrom the rails in the dorsal or ventral direction, suitable limit stopscan be provided. The dorsal limit stop is expediently connected rigidlyto the cover plate forming the seat (i.e. the guide rails). At theventral end, a limit stop should be provided which can be removed fromits limit-stop position so that the core can be more easily insertedafter implantation of the cover plate. The limit stop is then fixed inthe position in which it prevents the escape of the core.

[0009] Instead of a movement-limiting device which interacts with theouter edge of the core, it is also possible to provide one whichinteracts with an inner edge of a recess of the core. For example, onthe face directed toward the cover plate holding it, the core can have arecess which interacts with a projection on this cover plate. The recesscan have an elongate shape extending in the AP direction. In this case,it is expedient to design the articulation to be rotatable about thevertical axis. If the projection is made short in the AP direction or islimited in a circular way in cross section, so that it can pivot inrelation to the recess, the core is able to pivot, relative to the coverplate holding it, with respect to the vertical axis, so that apossibility of rotation of the prosthesis articulation about this axiscan be dispensed with. This allows for greater freedom in the design ofthe articulation. If it connects the core pivotably to the upper coverplate, the AP direction of the core is then determined by the APdirection of the upper cover plate.

[0010] A particular aspect of the inventive concept resides in the factthat a system of intervertebral prostheses includes, in addition tothose which have the described AP mobility, also other types, preferablyof corresponding external configuration, which do not have AP mobilitybetween the prosthesis core and the cover plate holding it. This allowsthe physician to decide, during the operation, whether or not he wishesto provide AP mobility. The cover plates of the prostheses movable orimmovable in the AP direction are expediently of uniform configuration,and only the core is different. However, provision can also be made forthe prosthesis core and the cover plate forming the articulation with itto be uniform in all types, while the AP mobility is afforded bydifferences in the cover plate holding the prosthesis core. Finally,there is also the possibility that all three components are uniform, andthat only the limit stop limiting the ventral movement of the prosthesiscore in the AP direction is differently located.

[0011] Where the terms lower and upper cover plate are used here, thisis not intended to imply that the cover plate forming the seat for thecore would always have to be arranged at the bottom. Rather, thearrangement can also be chosen the other way round. The claims thereforetalk more generally of a first cover plate and a second cover plate.

[0012] In order to prevent lifting of the core from the cover plateholding it, provision can be for the projection and the recess to bedesigned with interacting undercuts.

[0013] Preferred embodiments are explained below with reference to thedrawings, in which:

[0014]FIG. 1 shows a frontal cross section,

[0015]FIG. 2 shows a sagittal cross section,

[0016]FIG. 3 shows an exploded view of a first embodiment,

[0017]FIG. 4 the exploded view of a second embodiment,

[0018]FIG. 5 shows a variant of the lower cover plate belonging to theembodiment according to FIG. 4;

[0019]FIG. 6 shows two cover plates of a further embodiment withoutcore.

[0020] The lower cover plate 1 and the upper cover plate 2 of the firstembodiment have outer surfaces 3 and 4, respectively, which are intendedfor anchoring to the associated vertebral body. They are preferablyplane. However, other substantially flat configurations includingsuitable surface structures for better anchoring to the bone are alsoconceivable. The cover plates are preferably made of metal.

[0021] The lower cover plate 1 has a plane upper surface 5 facing towardthe upper cover plate 2 and enclosed on three sides by a collar 6 which,above an inner undercut, forms an inwardly projecting ridge 7. The lowercover plate 1 is of approximately rectangular shape in plan view. In thearea of its sides 8, 9, the branches of the collar 6 located thereextend parallel to one another and rectilinearly.

[0022] The upper surface 5 and the collar 6 of the lower cover plateform a seat for the prosthesis core 10, which is made of a material withgood sliding properties, for example polyethylene. It has a plane lowersurface which matches the surface 5 and which is delimited by an edgeridge 11 above which a groove 12 is situated. The ridge 11 engages inthe undercut of the collar 6 below the ridge 7. The ridge 7 engages inthe groove 12. Sliding play is provided between the collar 6 of thelower cover plate 1 and the edge of the core 10.

[0023] The core 10 has the same contour shape as the lower cover plate 1on both sides (right and left in FIG. 1) and dorsally (left in FIG. 2).The shape of its edge ridge 11 and of its groove 12 exactly follows theshape of the collar 6. Ventrally (right in FIG. 2), the core is slightlyshorter than the lower cover plate, so that play remains between itsventral end face 13 and the limit stop 14.

[0024] On their ventral edge, the cover plates 1, 2 each have a flange15, 16 which issues from them approximately at right angles and whichhas screw holes 17 for fastening to the vertebral body. Located in theflange 15 of the lower cover plate 1 there is a slot 18 in which alimit-stop plate 14 is held displaceably. It can assume the lockingposition shown in FIG. 2 in which it forms a limit stop for theforwardly directed movement of the core 10. It can also be pushed so fardown into the slot 18, or completely removed from the latter, as toallow the prosthesis core to be introduced easily between the coverplates from the ventral direction. It has two bores 19 which, in thelocking position of the plate 14, are flush with the screw holes 17.When the lower cover plate 1 is secured on the vertebral body via thescrew holes 17, the fastening screws also pass through the holes 19 andthus secure the plate 14 in its locking position.

[0025] The lateral branches of the collar 6 form a guide for theprosthesis core, in which guide said prosthesis core can move a certaindistance in the AP direction indicated in FIG. 3, namely by the width ofthe free space between the ventral limit-stop surface 13 of theprosthesis core and the limit-stop plate 14. The ventral part 21 of thecollar 6 acts as a securing limit stop which prevents the core fromescaping in the dorsal direction from the space between the cover plates1 and 2. The presence of the undercut on the collar 6 and on the edge ofthe core 10 is only of importance in the lateral areas 8 and 9 of thelower cover plate 1 and of the core, but not in the dorsal extent 21 ofthe collar 6.

[0026] At its top, the core 10 has a preferably convexly sphericalarticulation surface 22 which, in order to form an articulation,interacts with the concavely spherical slide surface 23 on the undersideof the upper cover plate 2.

[0027] Upon flexion movement, the upper cover plate 2 pivots slightlyclockwise in relation to the lower cover plate 1 in the view accordingto FIG. 2, and, upon extension movement, it moves in the oppositedirection. If the upper cover plate 2 exactly follows the directionpredetermined by the slide surfaces 22, 23, this pivot movement isassociated with a translation movement which is directed forwardly uponflexion (toward the right in FIG. 2) and directed rearwardly uponextension (toward the left in FIG. 2). Part of this translation movementmay be inconsistent with the physiological situation and may lead toundesired stresses. These stresses cause restoring forces which, in theprosthesis design according to the invention, result in the upper coverplate moving in the opposite direction relative to the lower cover plateand thereby compensating for the undesired component of movement.

[0028] Between the interacting guide devices of the core and of thelower cover plate, so much clearance can be left in the lateraldirection that a certain relative movement is possible also in thisdirection.

[0029] The extent of the movement clearance in the AP direction ispreferably between one and four, more preferably of the order of two tothree millimeters. If a relative mobility in the lateral direction isprovided, the extent of this should not be more than two millimeters.

[0030] In the second embodiment according to FIG. 4, the prosthesisconsists of a lower cover plate 31 and of an upper cover plate 32. Thelower cover plate has an upper, plane surface 33 on which the prosthesiscore 34 lies. Whereas this core in the first embodiment is guided at itsoutersides, in the second embodiment it has a recess 35 with undercutside edges 36 which interact with an elongate projection 37 of the lowercover plate with correspondingly undercut edges 38. The core 34 is thusmovable in the AP direction relative to the lower cover plate 31, in thesame way as was explained with reference to the first illustrativeembodiment. In addition, the interaction of the undercuts protects itagainst lifting from the lower cover plate. Suitable limit stops (notshown) can be provided which prevent the prosthesis core from escapingfrom the space between the plates.

[0031] The lower cover plate 31 can be replaced by the lower cover plate31 a which is shown in FIG. 5 and which differs from the lower coverplate 31 in that its projection 37 a is not elongate, but limited in acircular way in plan view. This means that the prosthesis core 34, whichis assumed to be connected in terms of rotation to the upper cover plate32 with respect to a vertical axis, can rotate about the projection 37 awithout impeding the desired AP movement. This may be desirable in thecase of an aspherical configuration of the slide surfaces between coreand upper cover plate.

[0032] This ability of the core to rotate relative to the lower coverplate can also be provided in the embodiment according to FIGS. 1through 3, by means of the edge 11, 12 of the core 10 being madecircular. It can then not only move in the AP direction between theparallel, lateral branches 8, 9 of the collar 6, but can also pivot.Instead of this, it is also possible for both the edge of the prosthesiscore and also the collar of the lower cover plate to be made circular.The core is then able to rotate relative to the cover plate, withouthaving mobility in the AP direction.

[0033]FIG. 6 shows only the cover plates of a further illustrativeembodiment, without the prosthesis core and without the devices whichallow the latter to move in the AP direction relative to the lower coverplate. This figure serves merely to demonstrate an embodiment of themeans which ensure that a prosthesis core held between the cover platescannot escape in the dorsal direction. These consist of one or moretongues 40, 41 projecting upward from the lower cover plate, and of oneor more tongues 42 projecting downward from the upper cover plate, whichtongues are offset relative to one another so that they each engage inthe space between or alongside the opposite tongues. The tongues are ofsuch a length in the vertical direction that, even with the greatestpossible pivoting of the cover plates, they do not move away from eachother to such an extent that the prosthesis core could escape frombetween them. Corresponding devices can also be provided at the sidesand at the ventral end.

1. Intervertebral prosthesis, in particular for the cervical spine,consisting basically of a first cover plate (1, 31) to be connected to afirst vertebral body, of a second cover plate (2, 32) to be connected tothe second vertebral body, and of a prosthesis core (10, 34) which isheld by a seat of the first cover plate (1, 31) and forms anarticulation with the second cover plate (2, 32), characterized in thatthe core (10, 34) is movable relative to the first cover plate (1, 31)at least in the AP direction.
 2. Prosthesis according to claim 1,characterized in that the seat has a movement-limiting devicecooperating with an edge (11, 36) of the core (10, 34).
 3. Prosthesisaccording to claim 2, characterized in that the movement-limiting deviceis designed as a guide device in the AP direction.
 4. Prosthesisaccording to one of claims 1 to 3, characterized in that the guidedevice is formed by two opposite, parallel lateral guide rails (7) forguiding in the AP direction.
 5. Prosthesis according to claim 4,characterized in that the lateral guide rails (7) are undercut, and thecore (10) has a ridge (11) engaging in the undercut.
 6. Prosthesisaccording to claim 4 or 5, characterized in that the first cover plate(1) has a dorsal limit stop (21) limiting the movement of the core (10).7. Prosthesis according to one of claims 4 to 6, characterized in thatthe first cover plate (1) has a ventral limit stop (14) limiting themovement of the core (10) and removable from its limit-stop position. 8.Prosthesis according to claim 2 or 3, characterized in that the core(34) has a recess (35) which cooperates with a projection (37, 37 a) ofthe first cover plate (31).
 9. Prosthesis according to claim 8,characterized in that the recess (35) has an elongate shape extending inthe AP direction.
 10. Prosthesis according to claim 8 or 9,characterized in that the projection (37, 37 a) and the recess (35) haveinteracting undercuts (36, 38).
 11. Prosthesis according to one ofclaims 1 to 10, characterized in that the articulation impedes orinhibits the rotation movement between the upper cover plate (2, 32) andthe prosthesis core (10, 34), and the core (10, 34) can pivot about thevertical axis relative to the first cover plate (1).
 12. System ofintervertebral prostheses, in particular for the spinal column,consisting basically of a first cover plate to be connected to a firstvertebral body, of a second cover plate to be connected to the secondvertebral body, and of a prosthesis core which is held by a seat of thefirst cover plate and forms an articulation with the second cover plate,characterized in that, in addition to the intervertebral prosthesistypes according to one of claims 1 to 11, the system includes types ofcorresponding external configuration which do not have AP mobilitybetween the prosthesis core and the first cover plate.
 13. Systemaccording to claim 12, characterized in that the cover plates of thecorresponding types with/without AP mobility are identical and the coreis different.
 14. System according to claim 12, characterized in thatthe second cover plate and the prosthesis core of the correspondingtypes with/without AP mobility are identical and the first cover plateis different.
 15. System according to claim 12, characterized in thatall three components are identical and a limit stop, which limits themobility of the prosthesis core in the ventral direction, is different.